This invention relates to a closed loop LPP ignition timing control for an internal combustion engine suitable for a motor vehicle. In such a control, the location, in crankshaft angle relative to top dead center, of peak pressure in the combustion chamber (LPP) is compared with a desired location of peak pressure (DLPP); and an engine ignition parameter such as spark timing is adjusted to return LPP toward the desired value. DLPP is generally about 15 degrees ATDC, although this may vary from one engine or engine operating mode to another.
An LPP ignition timing control of this type is shown in the U.S. Pat. No. 4,481,925, to Karau et al issued Nov. 13, 1984. In this system, the signal from a combustion pressure sensor is processed by a peak detector and an LPP signal generated to control engine spark timing in a closed loop feedback control. However, the accurate operation of a closed loop system as shown in Karau depends upon the presence of certain events and relationships which are generally assumed but are, unfortunately, not always present for all engine operating conditions. For example, a detectable peak combustion pressure must exist and be accurately assignable to a particular crankshaft angular position. The LPP point must not be varying too greatly or quickly for stable closed loop control. Changes in ignition timing must produce predictable changes in LPP and must be capable of bringing LPP to the desired value, DLPP.
Although these conditions are true for most operating modes of a typical internal combustion engine, they can not always be relied upon. In particular, conditions of high dilution from exhaust gas recirculation or lean mixture, greatly retarded spark and light engine load result in slower burning combustion with a low, flat combustion pressure curve. Since most internal combustion engines have a plurality of combustion chambers, fuel tends to be unevenly distributed among them, even with the most careful design. As combustion becomes more marginal due to a high average dilution level, the percentage of misfires becomes higher. This leads to wide variations in the locations of detected pressure peaks or an increase in the percentage of nonexistent or undetected peaks.
In addition, the relationship between LPP and ignition timing becomes adversely affected. In normal combustion the two are linearly related over most of the usable range of ignition timing, as shown in FIG. 2, wherein LPP in crankshaft degrees after top dead center is plotted as a function of spark advance or ignition in degrees before top dead center. Thus, a simple closed loop control is adequate to maintain LPP at the level marked TDLPP. However, in engine operating modes involving high fuel dilution and light engine loads, the curve of the relationship can appear as in FIG. 3, with a fold-over effect in the retarded spark region which prevents the desired value of TDLPP from being obtained at any spark advance value and further causes a reversal of the control relationship in the retarded spark area which requires modification of the closed loop system. These problems have not been addressed in the prior art.